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The Desert Margins – vulnerable to desertification, but not hopeless

dodoricoWe hear about increasing desertification, but how does it happen, and is it inevitable? Those are some of the questions that were answered by Paolo D’Odorico at a Columbia Water Center Seminar on April 23.

D’Odorico, of the Department of Environmental Sciences, University of Virginia, presented ‘Ecosystem Stability and Resilience at the Desert Margins,’ and discussed the ability of an ecosystem to return to a stable state after a disturbance such as a change in vegetation or a change in climatic factors, based on work in the Southern Kalahari. Stability can be viable plant cover, or stability can be irreversibly barren soil – desertification.

Most people are aware that vegetative cover, such as trees or other shade producing plants, helps to keep moisture in the soil, especially in the driest climates. Not all plants have the same effect, though.

One example studied was shrub encroachment into grasslands. When shrubs begin to displace soil-holding grasses, greater erosion occurs, resulting in clumps of plant-cover surrounded by bare earth, in turn leading to drying of the soil.

shrubdunes

In such a case, if fire is used as a land management practice, and used before all the grass seed stock is gone, it can pave the way for grass to return and reverse the process. Once the grass is completely lost, however, the process tends to be irreversible – stability of the barren kind.

In other situations, grass can be the problem. When a non-native grass invades an area, it can be less adaptable to changing climate conditions. During a drought, it can dry up and increase fire hazards. If the invader then burns, it can leave the land bare, again increasing desertification. In this situation the vegetation may not recover even if the rains begin again.

grassshrubs

In fact, fluctuations in rainfall between wet and dry may end up benefiting the ecosystem by encouraging native-plant biodiversity, and increasing the ability of the system to adapt to changes.  And that’s our preferred kind of stable state.

(For specifics see Ravi, D’Odorico, et al,Post-Fire Resource Redistribution in Desert Grasslands: A Possible Negative Feedback on Land Degradation, Ecosystems, 2009, or  Ravi and D’odorico, Post-fire resource redistribution and fertility island dynamics in shrub encroached desert grasslands: a modeling approach, Landscape Ecology, 2009)

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